References to any prior art in this specification is not, and should not be taken, as an acknowledgement or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other country.
The increasing sophistication of recombinant DNA techniques is greatly facilitating research and development in the agricultural industry. This is particularly the case in the horticultural area including the area of crop research. Of particular importance are the development of herbicide resistant plants and the development of pathogen resistant plants.
A number of approaches have been adopted to induce herbicide resistance in plants. For example, genes encoding enzymes, which deactivate or neutralize the active components of herbicides, have been expressed in plants. Whilst there has been some success in this approach, it is one component of a multi-disciplined and multi-strategy approach to maximizing yields of crops and products of crops and for maximizing returns from other activities within the agricultural and horticultural industries.
One of the major difficulties facing the agricultural and horticultural industries is the control of insect and other pathogen infestation of plants. Insects and other pathogens account for millions of tonnes of lost production on an annual basis. Although insecticides and other anti-pathogenic chemical agents have been successfully employed, there is a range of environmental and regulatory concerns with the continued use of chemical agents to control plant pests. Furthermore, the increasing use of chemical pesticides is providing selective pressure for the emergence of resistance in populations of pests. There is clearly a need to further investigate alternative mechanisms of inducing resistance in plants to pathogens such as insects, microorganisms, fungi, arachnid and viruses.
A range of genetic measures has been adopted in test trials. Whilst some success has been achieved, it is important for new and alternative genetic approaches to be developed to combat the difficulties of resistance.
One approach which has been suggested is the use of a group of proteins collectively known as “defensins”. The defensins have previously been known as γ-thionins and are structurally distinct from the α- and β-thionin families. Most defensins isolated and studied to date have been derived from seeds, especially those from Raphanus sativus and other members of the Brassicaceae family. Seed defensins are small (˜5 kDa) basic, cysteine-rich proteins and many have anti-fungal activity.
Over the last few years several cDNA clones have been isolated from the floral organs of solanaceous plants and Arabidopsis that encode proteins that are related to seed defensins. Unlike seed defensins, floral defensins are produced from precursor proteins that have an acidic C-terminal domain in addition to the defensin domain. The role of this acidic domain is unknown.
The defensin domain has little sequence in common with seed-derived defensins apart from eight cysteine residues that are strongly conserved. Although several cDNAs, which encode floral defensins, have been isolated, the corresponding proteins have not been isolated and their biological function has not been examined. The inventors have isolated both the cDNA and the corresponding floral defensin from the ornamental tobacco Nicotiana alata. They have determined that the defensin precursor is processed proteolytically to release mature defensin from the acidic C-terminal domain.
In accordance with the present invention, the inventors have determined that floral defensins have useful properties in inhibiting plant pest attack or infestation. Furthermore, a floral defensin from Nicotiana alata is shown to be particularly effective in controlling insect attack. The present invention also provides a new use of seed and known floral defensins in the control of insect infestation of plants.